Ferroelectric liquid crystal with positive dielectric anisotropy, chevron structure and grey scale

ABSTRACT

A method of driving a ferroelectric liquid crystal element which enlarges the effective cone angle. A ferroelectric liquid crystal is held between two substrates such that the helix is suppressed and a chevron layer structure is present. The ferroelectric material has a positive dielectric anisotropy. A first pulse is applied which causes switching from one state to the other, and a subsequent smaller AC pulse is applied which does not cause switching. The second pulse voltage control the effective cone angle, enabling grey scale.

Switching and display elements in which ferroelectric liquid crystalsserve as switching and display medium (FLC displays) are described, forexample, in U.S. Pat. No. 4,367,924. They comprise a layer composed of aferroelectric liquid-crystalline medium (FLC) which are enclosed on bothsides by electrically insulating layers, electrodes and boundary sheets,usually glass sheets. In addition, they contain a polarizer whenoperated in the guest-host mode and two polarizers when operated in thebirefringence mode. The electrically insulating layers are intended toprevent electric short-circuits between the electrodes and the diffusionof ions out of the glass of the boundary sheets into the FLC.Furthermore, at least one and preferably both insulating layers serve asan orientation layer which brings the FLC into a configuration at whichthe longitudinal axes of the molecules of the FLC are parallel to oneanother and in which the smectic planes are arranged perpendicular to orat an angle relative to the. orientation layer. In this arrangement, theFLC molecules can adopt two possible and equivalent orientations, intowhich they can be brought by applying pulses of an electric field. Ineach case, they remain in the last-generated orientation, even if thefield is switched off or the display is short-circuited. This means thatFLC displays can be switched between two stable states. The switchingtimes are in the region of μs and decrease with increasing spontaneouspolarization of the FLC used.

Compared with the previously used liquid crystal displays, which are allnon-ferroelectric, FLC displays have in particular the advantage thatthe obtainable multi-plexing ratio, i.e., the maximum number of lineswhich can be addressed in a time sequence ("multiplexing") isconsiderably higher than in the known non-ferroelectric displays.

A serious disadvantage of FLC displays is their hitherto too low opticalcontrast of multiplex-driven displays. In this form of addressing, themaximum possible switching angle is not fully utilized, but only aso-called effective switching angle θ_(eff). This is explained byundesirable twist states or by the presence of tilted smectic layers[see N. Hiji, Y. Ouchi, H. Takezoe, A. Fukuda, Jap. J. Appl. Phys. 27, 8(1988)]. In order to improve the contrast, special addressing modes havebeen developed, which in the case of dielectrically negative FLCmaterial (i.e., LC material having a Δε<0) lead to a widening of theeffective switching angle θ_(eff) [see T. Umeda, T. Nagata, A. Mukoh, Y.Hori, Jap. J. Appl. Phys. 27, 2187 (1988) and Y. Sato, T. Tanaka, M.Nagata, H. Takeshita, S. Morozumi, Proc. 6th Intl. Display Res. Conf.1986, p. 348].

Although this method produces certain improvements in the contrast, theyare by no means sufficient. In addition, undesirable side effects areobserved especially when the attempt is made to maximize the effect ofcontrast improvement by suitable addressing. These undesirable effectsare irreversible transformations in the geometry of the FLC cell,so-called textural transformations [see H. -R. Dubal, C. Escher, D.Ohlendorf, Proc. 6th Intl. Symp. on Electrets, Oxford, England 1988, p.334]. These transformations are unfavorable because they considerablylengthen the switching times of the displays.

Surprisingly, it has now been found that, even in the case ofdielectrically positive FLC material (FLC material having a Δε>0), theeffective switching angle can be enlarged and thus the contrastincreased by suitable multiplex addressing. A particular advantage ofthis method is that a textural transformation does not occur or at leastonly very slightly even when the effect is maximized.

A liquid-crystalline material which forms an S_(C) ^(*), S_(F) ^(*),S_(G) ^(*), S^(*) or S_(J) ^(*) phase proves to be particularlysuitable. Another particularly suitable material is an FLC materialhaving N^(*) phase in a temperature range above the ferroelectric phase,since in this case the orientation properties of the LC material areimproved.

The construction of the liquid crystal element according to theinvention is illustrated by means of FIGS. 1 and 2. The liquid crystalelement 10 comprises two transparent sheets, (12, 12') and an interstice(16) bordered by sealing material (14). The transparent sheets are madeof glass, plastic or similar material. The liquid-crystalline materialis poured into the interstice (16). On the inner surface of thetransparent sheets, electrodes (18) which are also transparent and arecomposed, for example, of ITO are present. On both sides of theinterstice, orientation layers (20) are also present. Structures whichserve as spacers (22) are, for example, those of spherical shape anduniform diameter, which are uniformly distributed between bothtransparent sheets 12 and 12'. The ferroelectric, liquid-crystallinematerial is oriented by orientation layers which are aligned by rubbing.

When the chiral, smectic liquid crystal material is poured into a thincell, the helix is unwound. When a single-axis orientation layer isused, a chevron structure is formed, which is characteristic of tiltedsmectic liquid crystals, in particular in the case where the liquidcrystals, upon cooling to form the tilted smectic phase, pass through anorthogonal smectic phase. In the smectic C^(*) phase, the molecules inthin cells have two stable states. In order to prevent the formation ofa chevron structure, it is, for example, possible to use an orientationlayer obtained by vapor deposition of silicon oxide at an angle, whichis however costly and leads to relatively long switching times.

Liquid crystal molecules have the tendency to align themselves parallelto the transparent sheets (except in the case where strongly tiltedorientation layers are used). Owing to the formation of the chevronstructure and the tendency of the molecules to align themselves inparallel, the angle between two stable molecular states is smaller thanthe original cone angle of the liquid-crystalline material. This angleis called effective cone angle 2 θ_(eff).

The most important feature of the liquid crystal element according tothe invention is that the effective cone angle can be widened withoutany irreversible change in the texture. It is also possible to addressintermediate values of the cone angle 2 θ_(eff) selectively by thevoltage applied, making it possible to produce gray steps.

In FIG. 3, the voltage applied to the LC element is plotted againsttime. A pulse having a high voltage (V_(s)) and a length τ_(s), has theeffect that the molecules can be switched from one state to the other.The subsequent, smaller alternating current pulses (V_(ac)), which havea length τ_(ac), which has the same or shorter length, stabilize themolecules but do not allow further switching.

The invention is illustrated by the example below:

EXAMPLE 1

A mixture M2 comprising the following two components ##STR1## has thefollowing phase sequence ##STR2## This LC mixture is aligned in theliquid crystal element above by means of an orientation layer rubbed ina parallel direction. The effective cone angle 2 θ_(eff) is 10 degreesat 38° C., Δε is just +0.6 at 10 KHz

    (Δε=ε.sub.| -ε.sub.⊥).

A shown in FIG. 5, the effective cone angle 2 θ_(eff) increases at anapplied voltage V_(ac) of more than 15 Volt in approximately directproportional to the voltage. Compared with liquid crystal elementsequipped with negative dielectric LC mixtures (negative Δε), evenvoltages greater than 25 Volt can be applied to the liquid crystalelements according to the invention without any sudden change in thetexture. Changes in the texture of liquid crystal elements containingdielectrically negative ferroelectric liquid crystal material have beendescribed, for example, by H. -R. Dubal et al. Proceedings of the 6thInternational Symposium on Electrets, Oxford, England, 1988, p. 334-338.

In LC mixtures having a negative Δε, the observed change in texture canbe explained by a deformation of the layer structure. The layerstructure of dielectrically positive LC mixtures is not subject todeformation.

In dielectrically positive LC mixtures, the molecular movement can beillustrated by rotation of the director on the curved surface of a cone,as demonstrated in FIG. 6. The optical properties of the LC element aredetermined by the effective alignment of the molecules, which herecorresponds to the projection of the molecules onto the plane of theglass of the LC cell. The molecules align themselves parallel to theglass sheets. Owing to the formation of a chevron structure in the LClayer, the axis of the smectic tilted cone is tilted relative to theglass or orientation layer, which leads to a reduction in the effectivecone angle. If now, as indicated in FIGS. 6, 7, a voltage is initiallyapplied which is large enough for allowing free rotation of themolecules on the curved surface of the cone and then such a voltage thatthe molecules can no longer rotate but are still stabilized, this stateis distinguished by an equilibrium between the electric torque and thatof the orientation layer. This results in an enlargement of theeffective cone angle.

The liquid crystal element according to the invention, which contains adielectrically positive LC mixture as the ferroelectric smecticmaterial, has the great advantage that, due to a large cone angle it hasgood contrast and moreover no irreversible changes in texture atelevated voltages, as a result of which the contrast is not diminishedeven upon extended use and frequent switching.

COMPARATIVE EXAMPLE

To demonstrate the advantage of the liquid crystal element according tothe invention, a dielectrically negative mixture M1 comprising threecomponents is prepared: ##STR3## This mixture has the following phasesequence: ##STR4## At 10 KHz, Δε is -0.8 and the effective cone angle 2θ_(eff) is 10 degrees.

The dependence of the effective cone angle on the voltage V_(ac) appliedis shown in FIG. 4. The threshold value of the voltage depends on theabsolute value of Δε; if a lower threshold voltage is desired, theabsolute value of Δε must increase.

As can be easily seen from FIG. 4, the effective cone angle changes form10 degrees to more than 35 degrees within a relatively narrow voltagerange (here from 7 to 10 Volt ).

Above 9 Volt, an increase in voltage does not lead to any furtherwidening of the effective cone angle; an irreversible change in textureoccurs. This phenomenon of negatively dielectric LC mixtures is causedby a twisting deformation of the liquid crystal film.

The changes in texture upon applying a higher voltage leads to defectsin the liquid crystal phase of the switching element with increasingtime of operation, which defects have a detrimental effect on thecontrast and increase the switching times.

We claim:
 1. A process for enlarging the effective cone angle in aliquid crystal element comprising a housing, which contains aninterstice bordered by a sealing material between two transparentsheets, and additionally at least one polarization film, at least onerubbed orientation layer, furthermore two electrodes on both innersurfaces of the transparent sheets and a liquid-crystalline,ferroelectric smectic material present in the interstice, wherein theliquid-crystalline material firstly forms a chevron structure, andformation of a helix is suppressed, and secondly has spontaneouspolarization and a positive dielectric anisotropy, said processcomprising firstly applying a pulse of a high voltage (V_(s)) and alength τ_(s) which has the effect that the molecules can be switchedfrom one state to the other, and secondly applying subsequent smalleralternating current pulses (V_(ac)), which have a length τ_(ac), whichis the same or shorter than the length of the first pulse, and which donot allow further switching.
 2. The process as claimed in claim 1,wherein the liquid-crystalline material forms an S_(c) ^(*), S_(F) ^(*),S_(G) ^(*), S_(I) ^(*) or S_(J) ^(*) phase.
 3. The process as claimed inclaim 1, wherein the liquid-crystalline material has an N^(*) phase in atemperature range above the ferroelectric phase.
 4. The process asclaimed in claim 1, wherein grey steps can be controlled with the aid ofthe effective value of the voltage.